Coal, formed from decomposed and compressed vegetable matter, is typically found in substantially horizontal seams extending between sedimentary rock strata such as limestone, sandstone or shale. Surface and underground mining are the primary techniques used to recover this coal.
Surface or strip mining involves the removal of material, known as overburden, overlying a coal seam so as to expose the coal for recovery. In recent years, surface mining has gained prominence over underground mining in the United States. This is due to many factors including:
(a) the increased material moving capacity of surface or strip mining equipment;
(b) lower costs for surface mining than underground mining;
(c) the better safety record of surface mining versus underground mining; and
(d) the higher coal recovery percentage for extraction of many coal reserves by surface mining.
Surface mining does, however, have its limitations despite these cited advantages. The primary limiting factor relates to the depth of the overburden. Once the coal seam reaches a certain depth below the surface, the amount of overburden that must be removed to reach the coal simply makes strip mining economically unfeasible.
When this occurs, large quantities of coal may still remain in the ground. If economic recovery of this coal is to be achieved, other mining methods must be utilized. Underground mining application in such an instance is, typically, very limited. This may be due to a number of factors including the existence of poor roof support conditions, the thinness of the seam and/or the presence of insufficient quantities of coal to warrant the large capital investments characteristic of underground operations.
Due to these considerations, auger mining is often used to recover coal following a strip mining operation where the overburden becomes too costly to remove. A large auger is used to bore into the face of the seam and recover the coal from beneath the overburden. Advantageously, auger mining is very efficient providing more tons per man per day than any other form of state of the art mining techniques: Auger mining may also be initiated quickly and requires a relatively low capital expenditure when compared to surface and underground mining. Auger mining has also been found to date to be the best method to use in relatively thin seams. Further, auger mining is safer than both surface and underground mining. Thus, auger mining may be used to effectively supplement a strip mining operation and recover small coal deposits that would otherwise be left behind.
Auger mining is, however, also not without its disadvantages. Auger mining provides a relatively low total coal recovery. Coal recovery for the resource area being augered is usually less than about 35%. Some of the lost recovery is due to the pillars of coal that are left standing to support the overburden between adjacent auger holes. The majority of the recovery shortfall, however, is due to the limited penetration depth achievable with even state of the art auger mining equipment.
More particularly, as penetration depths increase, a greater number of auger flights are required to convey the coal from the cutting head to the seam face for recovery. Each flight adds to the frictional resistance to the turning of the auger through contact with the walls of the bore hole. Additionally, the longer the string of auger flights, the greater the weight of coal being moved by the flights at any one time. As a result, it should be appreciated that auger power requirements increase rapidly with the depth of auger penetration.
Due to the above considerations, holes drilled by conventional augering equipment are usually only of a depth of 150 feet with 200 feet being rarely attainable. Of course, any increase in this figure is desirable as it would greatly improve the coal recovery rate from a resource area.
A mining system and method that was developed to meet this end is disclosed in U.S. Pat. Nos. 5,364,171, 5,261,729 and 5,112,111. These patents are all owned by the assignee of the present invention and the full disclosure of these documents is incorporated herein by reference. As shown and described in these patents and in FIG. 1, the mining system includes a continuous miner M for cutting coal C from a coal seam S. The cut coal C is fed by the miner M to a conveyor train T comprised of a series of modular conveyor units U serially connected end-to-end. This system allows mining to depths far exceeding the 150 to 200 feet possible with conventional auger mining equipment. In fact, depths of over 1500 feet have been reached.
Each conveyor unit U is supported on ground engaging wheels W so as to be adapted to follow the miner M as the miner advances into the coal seam S. A launch vehicle L is also incorporated into this system. The launch vehicle L includes a conveyor mechanism for receiving and conveying aggregate coal discharged by the conveyor train. The launch vehicle L also includes a guide track for supporting the end unit of the conveyor train T and a conveyor unit U to be added to the train. Further, individual drive assemblies are provided for (1) advancing/withdrawing the conveyor train with the miner and for (2) pushing the new conveyor unit into engagement with the conveyor train. Advantageously, the system allows the aggregate coal C to be cut and conveyed without interruption even when a conveyor unit U is being added to the train T. Hence, the system not only provides significantly improved recovery from the resource area but also operates more efficiently than augering equipment and provides improved productivity.
While the mining system and method disclosed in these patents has met with commercial success, the length of the launch vehicle (about 90 feet) has limited its application. Specifically, many existing benches adjacent an exposed highwall are not wide enough to accommodate a launch vehicle of this length. The present invention relates to a redesigned launch vehicle of reduced length (about 45 feet) that still accommodates the modular conveyor units of the mining system. Advantageously, such a launch vehicle allows the mining system to be utilized on relatively narrow benches for the first time.